An einzel lens, also called a saddle lens or a unipotential lens, is an electrostatic lens formed by three electrodes, a center electrode and two side electrodes. The center electrode is either connected to a ground potential or to a relatively low voltage potential. The two side electrodes are connected to a relatively high potential which usually is the anode potential. The focus of an einzel lens is slightly less sharp than that of a bipotential lens, but the einzel lens has the advantage that it does not require a second high voltage for a focus electrode. Einzel lens electron guns have been commercially used in color picture tubes, such as in the G.E. Portacolor, the RCA 15NP22 and the Sony Trinitron. The RCA 15NP22 had a delta electron gun and the G.E. Portacolor and Sony Trinitron used inline guns. The RCA and G.E. electron guns had individual tubular electrodes as the center and side electrodes in the paths of each electron beam. The Sony electron gun had large tubular electrodes as the center and side electrodes through which the three electron beams passed, crossing over each other at the center of the einzel lens.
Electron gun designs for use in large screen entertainment-type color picture tubes must be capable of generating small-sized high-current electron beam spots at the tube's screen. This requires a beam-forming region (BFR) in an electron gun which produces beams that can be easily focused and a main focus lens in the gun that has low aberrations. The beam-forming region of an electron gun comprises the cathodes, control grid (G1), screen grid (G2) and a portion of a focus electrode (G3) that faces the screen grid. An important requirement for a beam-forming region is that it produce beams having uniform current density across their cross-sections. Several new beam-forming region designs have been developed that accomplish such uniform current densities by selective prefocusing the center and outer parts of the beam in the G2-G3 region. When these new beam-forming regions are used with bipotential main focus lenses, very high performance can be achieved. However, the performance of such bipotential electron guns that incorporate the new beam-forming regions is extremely sensitive to misalignments in the beam-forming region. In some cases, this sensitivity is as much as seven times greater than is the misalignment sensitivity in more conventional electron guns. It appears that there are misalignment tolerances inherent in present manufacturing techniques that may render the electron guns using the newer beam-forming regions unusable. Therefore, there is a need for a new electron gun design that can take advantage of the newer beam-forming region designs by being less sensitive to misalignment in the beam-forming region.